Optical communications systems are well-known and typically include a transmitter system with a light source (e.g. a laser or laser diode) which emits a light beam. In a directly modulated optical communication system, the light source is directly modulated by an applied RF (radio frequency) signal such as a cable television (CATV) signal including many TV channels. In an externally modulated system, the light beam output by the light source is coupled to a light modulator (a phase or intensity modulator or both). The RF signal is applied to the electrical input terminal of the modulator which thereby modulates the light beam. The modulated light beam, in either case, is then coupled to a span of optical fiber, for instance 25 kilometers long or longer. The distal end of the fiber optical span is coupled to a receiver which detects the optical signal and extracts the RF signal.
In such optical systems it is a well-known problem that the carrier to noise ratio (CNR) at the receiver is lower than desired. This limits the length of the optical fiber span which can be used without use of a repeater (amplifier). Such systems are also prone to non-linear optical effects such as self-phase modulation, modulation instability, and SBS (stimulated Brillouin scattering).
Attempts to increase information carrying capacity include a system as in FIG. 1 with a wavelength division multiplexer (WDM). In this case, two transmitters 12 and 14 are each modulated by a different RF signal, designated RF1 and RF2. Transmitters 12, 14 each include a laser or laser diode directly or externally modulated by the RF signal. In this and the accompanying diagrams a single line connecting blocks indicates a conductor carrying an RF signal, while double lines indicate an optical fiber or optical coupler carrying a light signal. As is conventional, the term "frequency" herein refers to a radio frequency (electrical) signal, and the term "wavelength" refers to a light (optical) signal.
Transmitters 12 and 14 output light wavelengths respectively .lambda..sub.1 and .lambda..sub.2 which differ by a certain amount in terms of wavelength. Wavelength division multiplexer (WDM) 16 is for instance a commercially available component of a type well-known in telephony for carrying telephone signals on optical fiber. WDM 16 has a single output port which is coupled to a span of optical fiber 20. At the receiver end, receiver 24 typically includes a two receivers 24, 26 (photodiodes) sensitive respectively to wavelengths .lambda..sub.1 and .lambda..sub.2 and coupled to a second WDM 22 which convert the received light signals into output signals RF1, RF2. Hence in this system the two input RF signals RF1 and RF2 differ, and WDM 16 is used to increase system carrying capacity (the amount of information transmitted on a single fiber) so that the system carries the two RF signals RF1, RF2 instead of the usual one. It is to be understood that the RF signals RF1, RF2 are, for instance, telephone or multi-channel CATV. While this use of wavelength division multiplexing does increase the signal-carrying capacity of a single fiber, it does not improve signal quality in any way over a system which carries a single RF signal without the WDM 16 being present. In fact, undesirable effects such as cross talk are increased. The cross talk problem in analog communications systems is well known; see e.g. K. Kikushima, et al. Optical Fiber Communication Conference 1995, paper PD-24; A. Li et al., Electronics Letters, vol. 31, pp. 1538-9, 1995; and Z. Wang et al., IEEE Photonics Technology Letters vol. 7, pp. 1492-4, 1995. Hence while this is an example of use of a WDM, there is no resulting benefit in terms of signal quality improvement.